This invention is an automatic safety device for braking a vehicle which is moving in reverse. The need for such devices has been recognized for a long time. Many solutions to the problem of ensuring safety for vehicles when traveling backward have been devised. In the 19th century, for example, a simple and effective device was developed, consisting of several large springs, suitable for use on a wagon backing up to a loading dock.
The sophistication of such devices has gradually increased. Examples of devices that have been used include systems having a mechanical arm which applies the brake of the vehicle, or actuates a warning signal. The major disadvantage of such systems is that they require physical contact between the obstruction and the mechanical sensing means before the brake is actuated.
The present invention, instead of using a mechanical arm for detecting obstacles, uses an ultrasonic device which detects such obstacles before the vehicle can collide with them.
Ultrasonic ranging devices have been known in the art for some time. They have the advantages of simplicity and effectiveness, without some of the problems of radar (which could have adverse effects on pacemakers in populated areas, for example). Ultrasonic sensing devices depend on the fact that the human ear cannot detect sound waves below 20 Hz and above 20,000 Hz. Frequencies above 20,000 Hz are designated as ultrasonic. Such high frequency vibrations are generated by quartz crystal oscillators, exhibiting the piezoelectric effect. When the crystal is subjected to compression or extension pressure, it generates a voltage. Conversely, when the crystal is subjected to an alternating voltage, it expands and contracts at the same frequency as the applied voltage. When a crystal is made to vibrate at its resonant frequency, its amplitude of vibration becomes large, and the vibrations can be used as the source of compressional waves whose frequency is well above the audible level.
In ultrasonic ranging devices, short pulses of ultrasonic energy are transmitted directionally from a ceramic speaker and are reflected back by objects encountered by the pulses in a particular direction. The reflected pulse is detected by the speaker during "listening time", i.e. the time between transmitted pulses. Because the range is very short, the pulses last a few thousandths of a second. The longer the range, the longer the pulse. In the application of the present invention, ranges of up to six feet are used.